Some things just do not stack well
The old stack ‘em up trick…
This graph is renders beautifully. From the x-axis labels being shortened to the lack of chartjunk in the form of gridlines and excessive tick marks. The colors definitely make the chart stand out. I don’t think many would argue that point.
If you take a look at an earlier post, I featured another visualization taken from the same page of BusinessWeek’s January 28th issue. As you will see, the color scheme for the whole page was green and yellow.
This graph shows two points very well!
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The first point is the trend of “The Detroit Three” shown in green below. I can quickly and accurately see the initial jump, plateau and then gradual decline.
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The second point that is definitive is the total for both the Foreign series and The Detroit Three series. The total sharply rises until 2000 and then starts a gradual decline over the next seven years.
What is almost impossible to determine is the change in Foreign sales from year to year and over the entire sixteen years. Intuitively, I can see that after about year 2000 the yellow portion is larger because the total is about the same while the green section gets smaller. Simply put, the reason a stacked bar chart is a poor choice when comparing more than one series over time is due to the baseline not being the same for the second (yellow) series.
For this example, the point isn’t what the total sales were over sixteen years. The point is how one compare to the other. I know this from the title of the page being, Detroit is still behind, despite hard-won gains.
Point: When showing more than one series over time (time being the key here) the most logical choice should be a line graph. I’ve seen some horrid stacked bar charts with many more segments. On a rare occasion, I may use a stacked bar chart only when time or trending is not a factor.
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